This invention relates to a method and system for transferring finely divided particulate matter from a single source thereof to a plurality of discharge points, such as in the transfer of finely divided alumina to a plurality of aluminum reduction cells in a potline.
Primary aluminum is almost universally produced by the Hall-Heroult process wherein alumina (Al.sub.2 O.sub.3) is electrolytically reduced to aluminum metal in a fused fluoride salt electrolyte. Aluminum plants employing this process generally have a number of reduction cells (e.g., 80-160) electrically connected in series and either positioned side-by-side as with prebake cells or end-to-end as with Soderberg cells. For a more detailed description of the process, see The Chemical Background of the Aluminium Industry by T. G. Pearson (1955).
In the operation of aluminum reduction cells, small, discrete quantities of alumina are fed to each reduction cell to replenish the dissolved alumina which is consumed in making the metal. If the alumina dissolved in the fused salt electrolyte is reduced too much, an anode effect occurs where the resistance of the cell increases dramatically. Usually, alumina is added on one of three schedules, namely, at periodic intervals, when the resistance of the cell indicates that an anode effect is imminent, or when an anode effect occurs. But, in each instance, alumina is added only in small quantities, i.e., less than 200 pounds, usually less than 50 pounds, to avoid "mucking up" the cell (sometimes referred to as a sick cell) where undissolved alumina settles to the cathode-metal interface and thereby interfaces with the electrolytic process.
In most commercial aluminum plants, alumina is fed to the reduction cell either from a feed hopper permanently fixed to the cell superstructure or from a feed hopper mounted on a wheeled or tracked vehicle which moves from cell to cell. In the former instance, the feed hopper on the cell is filled with alumina from a bucket carried by an overhead crane. These prior methods have been less than satisfactory because they are inefficient, time-consuming, expensive and moreover, generate much dust in the potroom due to the frequent transfer of finely divided alumina from container to container.
Other methods of feeding reduction grade alumina to the reduction cell have been suggested wherein alumina is transported pneumatically or in some other fluidic form, such as with air activated gravity conveyors. However, such methods have not met with much commercial success, due in part to the difficulty in controlling the feeding of small individual quantities of alumina which are necessary for efficient reduction cell operation. Such fluidic methods are shown or described in U.S. Pat. Nos. 3,681,229; 3,006,825; 3,664,935; and 3,870,374.
It is against this background that the present invention was developed.